text:
Why does a T-tail produce a pitch-down moment in sideslip?
Here is something which I did not find in any book, but confirmed in several wind tunnel and flight tests: A T-tail causes a strong nose-down moment in sideslip. This can even be observed in a potential flow analysis, so no fancy viscous effects should be required for an explanation. But I have never understood why it happens. I would even have an explanation for a pitch-up, but the pitch-down really has me puzzled. Does someone here know more?

any correlation with the airfoil selected / incidence of the horizontal surface?

Fairings in the intersection area do modify the magnitude of the effect, but it shows up also in simple potential flow codes which use a flat surface for a wing (vortex at 1/4, control point at 3/4, two trailing vortices per panel). As for the incidence - my data is only for a trimmed configuration, so I have no data on different incidence angles.

FWIW I can't say I've experienced any kind of strong pitch-down moment until slipping very aggressively in the T-tail Katana. There was one time where there was a sudden pronounced pitch-down, but to be honest, that felt more like flow separation from the stabilizer. I suppose it's possible that humans simply tend to compensate without noticing, there is a lot of moving going on when entering a slip after all. Perhaps it's only noticeable in a wind tunnel where you can keep everything else constant?

@falstro: The pitch-down moment should build up slowly with sideslip, so you will just pull a little with moderate sideslip angles. Depending on the static stability, if you use a tape measure or ruler to measure stick position in flight, the effect should be detectable.

@PeterKämpf Does it also happen for a normal tail, just less strong? Or doesn't it happen at all with normal tails?

@ROIMaison: It does not happen with the regular tail position at the fuselage, but I have no data on horizontal tails halfway up (think Dassault Facon).

I'm thinking something along the lines of increased suction over the rudder on the left side (seen from the front) and increased pressure on the right side.
On the left side the suction cancels out with the increased pressure on the bottom of the elevator, whereas on the right side the increased pressure of the rudder amplifies the increased pressure of the elevator.
As pressure increases with $V^2$, the amplification would be stronger that the reduction, leading to a nose down force.
I'm not quite sure if this reasonable, but it might give people an idea

The endplate effect of an T-tail will make it a stronger in the case of T-tails, perhaps the presence of the fuselage completely prevents this effect for normal tails.

I have experienced this pitch-down effect in a Schweizer 1-36 sailplane.

I've found some useful research here: E.C. Polhamus. Some factors affecting the variation of pitching moment with sideslip of
aircraft configurations. Technical report, NACA T.N. 4016, 1958.
The image shows the situation for high angles of $\beta$:

At large angles of sideslip with swept horizontal
  tails, the loading will probably not be antisymmetrical mainly because
  of the difference in lift effectiveness of the leading and trailing portions
  of the horizontal tail caused by the difference in their effective
  sweep angles. This would result in a net lift induced on the horizontal
  tail which is a function of sideslip and tail height. This possible
  effect of tail height is illustrated in sketch 9 for large positive sideslip
  angles:

Interesting to see is that the model has no wings, so we can rule out any causes related to wings, in case people were thinking in this direction.
I putsome more thought into it, and I drew these diagrams, which helped me understand the things being said in the paper.
The blue pluses and minuses are the resulting velocities caused by the horizontal velocity as a function of the sideslip ($V_{\beta}$).
The red distributions are the forces on the elevator as consequence of this $V_{\beta}$.
This component creates an increased velocity over the left side of the rudder, and a decreased velocity over the right side of the rudder.
If there's no sweep, the effects are equally strong, and no effect on the lift is present (denoted by the top two images)
However, if there's sweep, the effect on the left side is less strong, whereas it is stronger on the right side (indicated by the two tails in the bottom row).
In a T-tail, this stronger influence is acting on the lower side of the tail. As it is a minus (meaning a relative reduction in velocity), it will lead to a reduction in suction on the lower side, or an upward force, causing a pitch down.
In a normal tail, the minus will act over the upper surface. It causes a deceleration of the flow over the elevator, thereby reducing the lift it generates. This causes a pitch up movement.

Polhamus only has an explanation for swept horizontal tails (which is simple and convincing). However, I found the effect even in unswept tails, so I am still not sure why it happens. OTOH, your answer is clearly the best  so far, so I accept it.

@PeterKämpf, I found a question which might help solve the problem here, direct link to image. For a T-tail the resultant force in side slip is left and up [=producing nose down]. whereas for a 'normal' tail it's left and down [=nose up].

Loss of aerodynamic lift of the horizontal tailplane (downward force) causes the aircraft to pitch down.
A disruption of airflow over the low pressure side of an airfoil has a greater effect on the airfoil's ability to create lift than a similar air flow disruption to the high pressure side of an airfoil.
On a conventional tail, the wind shadow of the vertical stab affects the high pressure side of the horizontal tailplane (the top surface) which is not as aerodynamically sensitive.
On a t-tail, the wind shadow of the vertical stab affects the low pressure side (lower surface) of the horizontal tailplane which is the aerodynamically sensitive side causing a greater loss of effectivity of the tailplane.

What disruption? If the effect shows up in potential flow codes, there is no separation involved. The effects on the windward side should compensate for those on the leeward side of the vertical, so in total the effects should cancel each other. And the whole thing happens also at higher AoA when the tail creates positive lift. I'm not convinced.

What disruption?!? you slip the aircraft, block the flow over the tailplane with the vertical stab and you say "what disruption?"

This is a rather drastical description for sideslip. The effect is linear, growing with sideslip angle well before any separation occurs. Again, it shows up in fully linear potential flow. If you still think this involves "disruptions", please reconsider.

Basically you are having a vertical fin that is lifting so one side of the fin has low pressure and the other has high pressure. So, there is compression in one side of the fin affecting expansion on the horizontal plane and you have expansion of the vertical fin affecting also expansion on the horizontal fin. The "distortion" or the "effect" of compresion over expansion is bigger of the effect of expansion over expansion. Netly there is lower downforce on the horizontal plain leading to pitch down.

From the test pilot notes of the F-104: the centre of pressure changes with vertical position of the tail. One of the reasons for implementing the T-tail on the F-104 was to create a more desirable sideslip-roll coupling, however the upward shift in centre of pressure also creates a nose-down pitching moment with sideslip.

Wouldn't the pressure on one side be balanced by an equal amount of suction on the leeward side? The pressure and suction on the vertical add up, resulting in a strong sideslip-induced rolling moment, but both should cancel on the horizontal tail, so the pitching moment is still a mystery to me. Also, the pictures imply a nose-up pitching moment change in case of a low tail in sideslip, but that does not occur. For a low tail, suction and pressure do balance, after all.


text:
Can I become a pilot if I am under treatment for Attention Deficit Disorder?
I know that before you even begin training in flight school, you are required to get a physical examination from a doctor to determine if you are fit to fly. Online articles stressed the importance of being completely honest with the doctor about any existing conditions.
I was just recently diagnosed with ADD (or rather, I described my issues to my regular physician and simply started receiving medication to treat it) and this is something I would naturally disclose at the physical examination.
Does FAA disqualify someone from getting their pilot's license on the basis of ADD?
I take medication for this once a day and it resolves any issues I face from it.

This is a great question.  I was denied a medical due to on-going use of Concerta for adult ADD.  I don't know if I have been misdiagnosed or if the medication helps me perform better at work for some other reason but I do know that I have been a successful software engineer, business owner, and UAV pilot for years.  I am not sure why the FAA feels that this issue is so binary as to merit blanket discrimination.

Concerta is implicated in cardiac events

@rbp So are hamburgers.

I'm going to assume you're asking about US/FAA requirements - If not what's below may not be particularly helpful.
ADD is not a disqualifying condition, however if you have a formal diagnosis of ADD or ADHD you may need to undergo additional testing in order to receive a medical certificate.
Also some of the medications used to treat ADD/ADHD may be disqualifying (I know that Adderall, Focalin, and Ritalin were all on the FAA's "No Fly" list - I'm not sure if they're still an absolute NO or if you can get approval for them).
AOPA has a pretty decent drug list that you can search to see if what you've been prescribed is permitted - it's a little easier to search/cross-reference than the official FAA "Do Not Issue/Do Not Fly" list and they do a good job keeping it up to date, and a brief article on ADD/ADHD & medical certification.
They also have a service to assist you with getting your medical (as part of Pilot Protection Services) which can be helpful.

The question is regarding US/FAA requirements, thank you for pointing that out. I've added a clarification to the question.
I haven't had a formal diagnosis, but I am taking one of the medications you list there. I tried accessing the AOPA site, but it looks like I need to be a member to view it.

From the linked AOPA article, "the FAA does not currently allow medical certification for a diagnosis of ADD/ADHD requiring the use of medication." That seems pretty clear and conclusive.

@raptortech97 That article is a few years old - the situation may be different now, but I think all the common ADD/ADHD meds are still on the banned list. This site has a medication reference that doesn't require a membership & everything it turns up for ADD/ADHD is disallowed.

@voretaq7 Can you update your answer to include the medication reference link? I was able to use that to look up my medication directly, and it provided me a clear and definite answer that it was against FAA regulations. I'll accept the answer after that.

The FAA "Do Not Issue - Do Not Fly" list is located at: https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/ame/guide/pharm/dni_dnf/. ADD/ADHD drugs are included on the Do Not Issue list.

@Neuty I've updated it to include newmanth's instead. (There's an FAA Official List somewhere as well but Google was NOT cooperating this afternoon!)

PLEASE NOTE that some of the information here is wrong.  Recently (within the last 18 months?) the FAA has changed its policy to permit the use of two specific drugs (used independently of each other) to treat ADD/ADHD.  Regardless of this good news, your path to a medical certificate will be difficult and possibly expensive.  Your best bet is to contact a specialist in difficult medical certifications.  One such person is found at www.aeromedicaldoc.com

@SkipMiller Do you have a reference for the permitted meds? Literally everything I've found (from both the FAA and the usual alphabet soup pilot groups) seems to indicate that if you're on any of the "standard" ADD/ADHD meds you're effectively grounded, but if there's an authoritative source saying otherwise I will happily update the answer to reference it.

@voretaq7 I have decided not to answer your question.  I am not a medical professional and the last thing I want is for some pilot to act on information from a non-professional source when the solution is in fact very complicated.  I have posted the website of the individual who authored and sheparded this narrowly limited proposal through the FAA Medical hurdles.  Anyone who needs to know the facts of this change and how it affects his/her specific situation is well advised to contact a specialist in difficult medical certifications.

@SkipMiller I believe you are incorrect. I have looked at the relevant laws for the US and there are currently no ADD/ADHD medications which are permitted as of 2018. This is according to the list of approved medications. Perhaps you are thinking of Bruce getting one or two of the exceptionally rare exceptions for Ritalin?

You can get a Pilot certificate under the Light Sport Category. This does not require a medical, only a current Drivers license. I've never seen a doctor for flying and I've had my ticket for a few years. You will be limited to VFR flight and aircraft size restrictions.(small plane, two seats to name few restrictions) But there are lots of options for Light Sport Aircraft, and many more manufactures getting on board with new equipment. Your CFI will teach you almost everything a beguining private pilot learns, because you have to be able to operate in the same airspace system with the rest of the VFR world. The training time requirements are less than PP and many pilots are starting with Sport and then moving up.

I know a lot of older pilots don't know much about Light Sport and there is a stigma regarding that new entry level Flight option. As I mentioned, the minimum flight time requirements are less and you can get Sport with a minimum of 20 hrs ( the average is going to be more like 30hrs) but it still gets a student in the air faster and cheaper than private.  Here is a simple search to show the wide variety of aircraft. https://www.google.com/search?q=light+sport+aircraft&biw=768&bih=928&tbm=isch&tbo=u&source=univ&sa=X&ei=zbARVcn_Fob6yATl_YDQCQ&ved=0CDUQ7Ak

You wrote that it was only your normal medical doctor that wrote the prescription.  I would go back to him and verify that he did not write an official diagnoses of ADD in your medical records.  And if he did, I would ask him if he is willing to amend the record and call it "possible ADD".  I am pretty sure he would not have written that you have a diagnosed ADD condition as it takes more than a conversation with a general practitioner to make such a diagnosis.
Assuming there is no official diagnosis of ADD in your record, all you have to do is get off the medication, wait three days (instead of 90 as folks with an official diagnosis have to) and then take the three neurotests.  You will not have to find a psychologist who is willing to "un-diagnose" you.
See https://www.aopa.org/go-fly/medical-resources/health-conditions/mental-health/add-and-adhd for more details.

Be forewarned: I hold a multi-engine instrument rating but can no longer fly due to the loss of my medical certification.
I attended Embry-Riddle Aeronautical University through an extension campus at Hillsboro Aero Academy. I completed all of my flight tests and exams in the minimum required time. I had the entire staff behind me when confronting the FAA on the matter of ADD.
Unfortunately, the FAA's requirements to get a waiver are quite strict (which includes taking the Wisconsin card sorting test before acquiring a medical certification). According to the three psychologists that I visited in trying to acquire a waiver, the tests are difficult to pass for even the most focused individuals.
My recommendation is to not say anything. I didn't at first and that's how I got so far. It was only when trying to get a class 1 medical certificate that I admitted to having once been diagnosed with ADD. I hadn't taken any medication in five years at that point and had no symptoms, but few psychologists will un-diagnose you due to the severe liability it could pose on them and others. I spent thousands of dollars trying to acquire a waiver and invoked the AOPA legal insurance that I had, but nothing helped.
The FAA's job is to deny you. Until these things are looked at more seriously by the FAA, I would suggest not saying anything.

Hi, and welcome to Aviation SE! You clearly have first-hand experience with this issue, and that's great. However, there are some issues with this answer. For one, it reads more as a rant and a personal story than an answer. It also seems to answer "what should I do" more than "can you become a pilot if you take ADD medication." Also, intentionally failing to disclose one's diagnosis during a medical is illegal, and we ask that people don't advocate breaking the law on here. You might want to look around to see the kind of answers we encourage here. I'm sure this can turn into a great answer!

Welcome ADD pilot. I feel sorry for your situation and I understand why you write the way you do but I fully concur with @raptortech97. This is too much of a rant so I have to vote to delete the answer. Let this not discourage you to post another answer or improve this one, I am sure your experience is of great value to others.

I have to admit, this is a very honest answer, ADD pilot. Likewise, I don't condone lying to federal regulations officials or breaking the law, but I think the answer retains some value as a cautionary tale for pilots who might be facing similar circumstances.

Suggesting that someone lie on their medical application (including omitting things they're required to disclose) is supremely bad advice - it is also a felony (18 U.S. Code § 1001). Pilots have been sent to jail for this.

I am in a very similar situation as you. If you have ADD, my initial advice is "good luck!" My process went as follows:
First, I had to go to my doctor and asked to be officially taken off of the medication. Next, I went through a 90 day waiting period. During that 90 day waiting period, I met with my AME. Your AME must be HIMS certified. He gave me my physical and all other aspects of my health checked out. Out of pocket cost for physical and using him as my case manager was \$600.
After the 90 days, I took my aviation neuro-psychological exam. This exam usually lasts 7-9 hours and insurance does not cover it. My out of pocket cost was \$2500. Within 24 hours of my neuro-psychological exam, I had to have a urine drug test done. This test must be done under direct observation; otherwise it is not valid. This only cost \$100.
About a week and a half later, I met with a psychiatrist. This was about a three hour meeting that cost me \$550 out of pocket. This test was to determine my personality, check for alcoholism, depression, etc.
Unfortunately, I was still not done after that. The way the neuro-psychological exam is set up is that if you don't do at least average on one section, then you need cognitive therapy. My scores were superior and high average on all but one section and I got the therapy recommendation. They gave me an IQ test in which I scored a 138. That's an exceptional IQ, but somehow I cannot pass this test.
Right now I'm waiting for my AME's recommendation and my the psychiatrist results. Pretty sure I'll need therapy. I've been in flight school since July and my CFI has been ready to let me solo for months now. The whole process is pretty screwed up. Looking back, I should have kept my mouth shut.

"Should have kept my mouth shut..." and committed a felony by not disclosing the known medical condition? Very, very, VERY bad plan, as the comments on the other post with similar advice make clear.

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text:
Would I hear the 90Hz and 150Hz signals when tuned to ILS frequencies with a basic handheld radio?
If I were to make a basic handheld radio and set it to listen to any ILS, would I hear the frequencies which tell you whether you're left or right of the center line?
On similar note, how come pilots don't hear the 90/150Hz frequencies but rather the Morse code identifier for the ILS?

There is no need to hear the 90/150 Hz frequencies.  If they are not there, the indicator would either display nothing or full range deflection to one side.

In short

Would hear the 90Hz and 150Hz signals?

You would hear them, mixed and very attenuated. If you need to decode them correctly you can use a software-defined radio.

how come pilots don't hear the 90/150Hz frequencies but rather the morse code?

The localizer receiver (VHF) receives individually three pieces of information: 90 and 150 Hz tones, and everything between 300 and 3000 Hz.

This latter range includes an ident code on 1020 Hz and an optional voice transmission used by ATC as a fallback to contact a pilot when their regular communication receiver is failed. This range is fully routed to the audio speakers.

The two tones on 90 and 150 Hz are used to drive the azimuth information of the ILS indicator, they are not routed to the speakers.

The glide slope receiver (UHF) receives only the two tones to drive the vertical indicator of the ILS. This receiver is not connected to the speakers.
Details follow.

Localizer transmitted signal
The localizer signal is a complex signal which SBO and CSB components are transmitted individually and interfere in space to produce 90 and 150 Hz which proportion varies according to the angle with the centerline. If you need to understand the spectrum of these signal, see How is varying modulation depth achieved by localizer ground transmitters?
When these two interfering signals are received together they look like a DSB-AM signal modulated with two tones, 90 and 150 Hz:

Source.
Tones can be demodulated using a VHF AM receiver (alternatively a SSB receiver). However this demodulation must be tailored to the signal carried.
In DSB-AM the two sidebands are mirrored on each side of the carrier and only half of the available bandwidth can be used to carry the modulation. For a channel spacing of 8.33 kHz, only 4 kHz are available, and in fact a standard VHF AM communication receiver is optimized to receive voice and only extract audio frequencies between 300 Hz to 3000 Hz (ICAO specifications), the significant frequencies of the human voice. The extra portions are attenuated to limit noise addition.
The ILS transmitter uses the available bandwidth to carry information at discrete audio frequencies: 90 Hz, 150 Hz and, for the identification, 1020 Hz.
AM Communication receiver
Assuming it is able to receive the localizer channel frequency, a standard communication receiver can extract the ident information, but will attenuate 90 and 150 Hz.
Still you'll receive the two ILS tones, mixed, with a prominence of one depending on the position of the receiver.
Optimized reception using a SDR
If you want to extract the tones correctly you likely need a software defined radio (SDR). Standard radio receivers are still used, but they are hardware, meaning buying components and using a soldering iron. With a SDR, the appropriate digital filters are programmatically defined at 90 and 150 Hz with steep edges. Such applications are common. Here is an example for ILS, based on the open source GNU radio.
Use of audio signals by ILS receivers
The aircraft ILS receiver filters portions of the sidebands to separately detect 90 Hz, 150 Hz, and 1020 Hz.

It routes 1020 Hz of the ident to the audio speaker for the crew. If an ATC voice transmission is superimposed to the localizer signal, it occupies the range 300-3000 Hz, and in fact this whole range is routed to the speaker.

90 and 150 Hz are compared to determine the difference of depth of modulation and to show it on the ILS indicator.

Two guidance signals must be processed separately, the localizer sent on a VHF channel and the glide slope sent on a paired UHF channel. There are actually two radio receivers.

The Wikipedia article seems to suggest that you could, with the right filters:

A receiver in front of the array will receive both of these signals mixed together. Using simple electronic filters, the original carrier and two sidebands can be separated and demodulated to extract the original amplitude-modulated 90 and 150 Hz signals. These are then averaged to produce two direct current (DC) signals. Each of these signals represents not the strength of the original signal, but the strength of the modulation relative to the carrier, which varies across the beam pattern. This has the great advantage that the measurement of angle is independent of range.[4]

Bold-formatting added.
As for what pilots hear, there's no advantage to us hearing the 90/150 Hz signals, which would be basically a buzz. But there is good reason to hear the ident, so you know you're receiving the correct station (and it isn't transmitting the T-E-S-T identifier, signifying that it's undergoing maintenance & shouldn't be use to navigate with), so the receivers are set up so that's what - and all that - we hear.

The Morse code signal is at 400Hz. There is a band pass filter on the audio output that filters out the 90Hz and 150Hz and lets you hear the 400 Hz Ident.

@Gerry Just checked the FAA.gov website and they list that both 400 and 1020 Hz tones are valid for Ident signals on NDBs. I didn't see anything specifically mentioned about the frequency when used on VOR, just that it may have an Ident signal and it might use TEST when the beacon is not in operation.

@penguin359 You are correct. 400Hz is used for ident on NDBs. VORs and LOCs use 1020Hz. The ILS or VOR receiver has an internal band-pass filter for the nav signal and one for the audio. What goes thru the audio bandpass filter is sent to the audio output. On some newer receivers, the audio is digitally processed to decode the morse code. It can then send the ident information to the displays over a digital bus, typically ARINC 429.

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text:
What's the difference between taking priority and taking over the other sidestick in Airbus?
I know that on the sidestick, there is a button to disconnect the autopilot, and also to take control of the aircraft over the other side.
However, I'm confused about what the difference is between priority and complete deactivation over the other sidestick.
When the "Priority Left" or "Priority Right" alarm sounds, does that mean that the prioritized side will be more considered than the non-prioritized side? For example, the priority left button is pressed on the captains side- not HELD, but simply pressed. Would that mean that the Captains inputs would be more effective than the F/O's inputs?
Or would that mean that the F/O's control would be temporarily disabled (only whilst the button is being held)?

Normally, both sidesticks are prioritized equally and simultaneous input is added. While one pilot keeps their priority takeover pushbutton pressed, the other sidestick is completely deactivated. There is no "more effective" logic, it will ignore all inputs from the other stick (this refers to moving the stick, the buttons on the stick are still functional). Momentarily pressing the priority takeover button without holding it, will only deactivate the autopilot, but not result in any priority change.
The FCOM contains the full details:

Sidestick priority logic

When only one pilot operates the sidestick, it sends his control signals to the computers.
When the pilots move both side stick simultaneously in the same or opposite direction and neither takes priority, the system adds the
signals of both pilots algebraically. The    total is limited to the
signal that would result from the maximum deflection of a single
sidestick.

Note: In the event of simultaneous input on both sidesticsk (2° deflection off the netural position in any direction) the two green
SIDE STICK PRIORITY lights  on the glareshield come on and "DUAL
INPUT" voice message is activated.
A pilot can deactivate the other sidestick and take full control by
pressing and keeping pressed his priority takeover pushbutton. For
latching the priority condition, it is recommended to press the
takeover push button for more than 40 seconds. This allows the pilot
to release his takeover push button withou losing priority. However, a
pilot can at any time reactivate a deactivated stick by momentarily
pressing the takeover push button on either stick. If both pilots
press their takeover pushbuttons, the pilot that presses last gets
priority.
Note: If an autopilot is engaged, the first action on a takeover pushbutton disengages it.
In a priority situation:

A red light comes on in front of the pilot whose stick is deactivated.
A green light comes on in front of the pilot who has taken control, if the other stick is not in the neutral position (to indicate a
potential and unwanted control demand).

(Airbus A320 FCOM - Flight Controls - Controls and Indicators)
This logic likely has not changed in any subsequent fly-by-wire Airbus aircraft. The A380 FCOM has a different description, but the general logic seems to be the same:

SIDESTICK PRIORITY LOGIC
At all times, only one flight crewmember should fly the aircraft.
However, if both flight crewmembers use their sidesticks
simultaneously, their orders are algebraically added.
The flight
control laws limit the combined order to the equivalent of the full
deflection of one sidestick.
A flight crewmember can deactivate the
other sidestick and take full control, by pressing and keeping pressed
the sidestick pb.
If a flight crewmember presses their sidestick pb
during 30 s, the other sidestick is permanently deactivated, until any
flight crewmember presses their sidestick pb.
If both flight
crewmembers press their sidestick pb, the last pilot to press gets the
priority.


text:
How was the 747 and DC-10's trimmable stabilizer actuated?
I was recently reading about Alaska Airlines Flight 261, a crash where understanding it in detail requires a bit of a deep dive into the trimmable stabilizer. On that particular aircraft (MD-80), it was electrically actuated using an electric motor and jackscrew.
I'm curious how this works on larger aircraft like the DC-10 and 747. Both of these suffered well-known loss-of-control incidents (UA232, JAL123) where the crew lost control of most or all flight controls due to catastrophic hydraulic damage (with these larger planes being simply too big to have purely mechanical fallback, they were designed with hydraulic redundancy that failed in the extreme cases of these incidents.) In UA232's case, no flight controls were available, while in JAL123's case, only an electric alternate system for the flaps was usable.
Presumably, this means that the stabilizer on these larger aircraft was hydraulically powered as opposed to electrically. My question is, how? Did they use a jackscrew mechanism that was driven by a hydraulic motor instead of an electric motor? Or did they use a different sort of mechanism altogether?

You've picked two (UA232 and JAL123) quite specific incidents. In both of them the tail cone got extensively damaged and any line passing through it would have been damaged, no matter if it had been hydraulic or electric and no matter how redundant it had been.

Alaska Airlines was actually a tale of grease, and the pitfalls of intermixing clay based and lithium based greases. It's a long and complicated story that affected the entire aircraft industry, because nobody was aware of the problems of intermix and there were no limitations, and the MIL-SPEC confusion made things worse.

Added Wikipedia links to the 3 crashes mentioned for those who aren't already intimately familiar with them. Not everybody walks around with encyclopedic knowledge of airline crashes and investigations rattling around their brain pan...

Affirmative, it's hydraulic. NASA-CR-159010 https://ntrs.nasa.gov/api/citations/19810018578/downloads/19810018578.pdf is a good reference: "The stabilizer control system uses a linear, ball-screw actuator, driven by two hydraulic motors..."
With all or most of the vertical tail gone and other roll issues they really needed help in the roll and yaw axes, also. Furthermore, while a usable stabilizer could have helped with pitch trim, it would have been challenging to say the least to use it to maneuver to a safe landing.


text:
How are aircraft shipped to their customers?
This is sort of the inverse of this question.
I was looking at the Boeing facility in Renton, WA in Google maps, and I realized, the airport is across the (Cedar) river from what I assume to be the construction facility. The Boeing facility in Everett, WA is also across Highway 526 from the airport. That prompted this question: How is an airplane that just finished getting built (in any facility) delivered to the airline that bought it?
This question is targeted at large passenger aircraft being delivered to airlines.

@DanPriest I would not say this is a duplicate. If planes are always flown to their customers, how do they take off from that Boeing facility? How do they get across the river? The Boeing Everett facility is across a highway from the airport there too.

See http://aviation.stackexchange.com/questions/3367/how-do-airliners-get-from-the-factory-to-the-client-if-the-aircraft-does-not-hav for a related question.

They are flown to their customers by either Boeing or pilots from the given airline.
As for the Boeing factory its self, the Everett Factory users Paine Field as their runway. If you look closely there is a bridge that crosses the highway and connects the plant to the main ramp of the airport. This appears to be painted for car traffic but is also used for aircraft. The Renton Factory lies adjacent to Renton Municipal which it uses to fly the aircraft in and out there is also a small bridge connecting them over the river.
Similarly the main airbus factory is located next to Toulouse–Blagnac Airport. Bombardier lies next to Montréal–Pierre Elliott Trudeau International Airport.

The bridge appears to be painted for both car and aircraft traffic—the centre-line is yellow and connects to the other yellow taxiway lines at the Paine Field side.

@JanHudec For what it's worth, in my experience driving in the United States, the center line of a two-way road is always yellow. But it's not usually thick and solid like the one on that bridge appears to be.

@TannerSwett, but airport convention is that yellow lines are for aircraft and white ones for ground stuff

Final assembly usually takes place at a location with its own runways or collocated with an airport, so once airframe assembly is complete, the aircraft undergoes its final inspections, test and acceptance flight and is then simply flown to a location specified by the customer, typically a hub or maintenance base.
To use the Everett facility as an example, it is adjacent to Paine Field (KPAE) and the assembly facilities have direct access to the airport environment.

Is there supposed to be more to this post? It seems to just end...

@kingledion no, there wasn't supposed to be. My brain must've broken while editing the answer and didn't spot it. Thx for pointing it out. Edit made.

In general the customer sends their own pilots out to take delivery of the airplane.  Usually they are immediately pressed into service.  When I was out at the Everett facility in summer 2000, we had customers who would pick up a 777 at KPAE, then make the short flight south to Sea Tac Intl right to the gate and immediately fuel and load it for a intercontinental flight.
Most aircraft factories are located at public airports for exactly that reason.  See the VFR sectional for the Wichita KS area.

Boeing does not ship airplanes to customers!  Getting the wings put on happens at Boeing, because they have the equipment & expertise to perform this crucial step correctly; that's not something you want being done after delivery. "Some assembly required" works fine for many things, but not for airliners!
When the airplane leave the factory, it is flown to the purchasing airline -- typically by that airline's own pilots, as discussed in this question.

I think the intent of the question was more along the lines of: how does the aircraft get from the factory to the airfield so that it can be flown to the airline's requested destination, since the facility and airfield appear to be separated by a river in one case, and a highway in the other.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Do helicopters ever dump fuel before landing?
In my answer to this question about fuel dumping I included the following rhetorical question:

Or if you're dumping fuel from a helicopter (does that ever happen?)
  it could hover over a tank on the ground, pump out the fuel and then
  land.

Two commenters said that helicopters should never need to dump fuel because if they can take off with a full fuel load then they can land again. Makes sense, but this news story refers to a helicopter dumping fuel in order to land, although it's only one article and the general (non-aviation) press is notorious for misreporting aviation incidents.
So, can/do helicopters ever dump fuel as a safety measure before landing?

Yes

07-NOV-1971
Sikorsky CH-124A Sea King (S-61B)
Suffered single engine failure on take-off. Pilot attempted to return while also dumping fuel.

From what I've read, fuel-dump systems are more common on military helicopters.
Sometimes, a helicopter may not have enough power to hover in ground effect for a normal landing or take-off because of loss of an engine, high density altitude, or being overweight. The alternative procedure is to perform a running take-off or running landing, which is an advanced, but not an emergency, maneuver.
But if the copter is landing in a confined area, such as a small pad on a frigate in a stormy ocean, there's no room for a running landing, so its better to dump fuel to allow the copter to hover into a landing.

Military copters are far more likely to lose power in flight than civilian ones.  Nobody's flinging shrapnel or bullets at civilian choppers.

can sometimes take off by flying forwards. Several effects come into play.  Reduced torque required on the tail rotor as speed increases since the tail assemblage provides lateral stability so more power to the main rotor.  "Translational lift" as air flow over the rotor disk increases really kicking in at about 20 kts.  If you accelerate slowly forward, without increasing collective pitch, the helicopter will start to climb as translational lift increases.  Reduced power as speed increases up to about 65 kts.  It's a standard manouveur taught during the PPL.

My 8th grade history teacher was a Huey pilot who had just returned from Vietnam. He said that the choppers leaving on combat missions were very heavily loaded, so on hot days they would have to use the forward momentum method to take off. They would do this by tying them to jeeps and pulling them down the runway.

Something we didn't know until the end of the cold war... the Soviet MI-24 Hind attack helicopter always did a running takeoff. It didn't have enough power to take off vertically. It could hover only for maybe ten seconds, before the over boosted  engines would be damaged.

Helicopters can take off when too heavy to hover, either by moving forward while in ground effect or by reducing the pitch of the anti-torque rotor to get a bit more power into the main rotor (there's a book called Chickenhawk by Robert Mason which describes using both of these processes on Hueys in Vietnam - not a bad read IMO). It's risky to use either of these methods in the first place, but there's added risk as landing overweight is so dangerous. So in helicopters which may have to take off overweight being able to dump fuel is a good safety feature.

"too heavy to hover" and "moving forward while in ground effect" are contradictory. you can't move forward in ground effect if you are not in a hover. what you're talking about (incorrectly) is a running take-off, and it is a normal (albeit advanced) maneuver, not an emergency maneuver, taught to every private pilot. (https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch10.pdf)

@rbp It reads clearly to me: You may be "too heavy to hover" outside of the ground effect zone.  So you hover in ground effect, then before leaving ground effect zone get forward velocity, then pull up.  Landing by doing the reverse is harder.

Define “ground effect zone”

I have never crewed on a helicopter that dumps fuel, nor have I known of any machine in the fleet that even has that capability - though I have been wrong before.
It has been noted already that helos can take off and land at max Gross Take Off Weight (GTOW), or all up weight (AUW) and thus do not require weight shedding to land safely (as we don't have wings to store fuel so we don't have to worry about over-stressing them on landing). However, keep this in mind: different fuel loads alter the emergency course of action in the case of a forced landing.
When considering helicopter performance (and we will talk in terms of torque), we explore 2 main ideas - power required and power available.
Power Required
Power required is the amount of torque that I need on the main rotor to overcome gravity and get into hover flight. Hover flight is always the most power hungry because i'm drawing air down into the rotor in order to create the smooth fast airflow over the blades and then pushing it down to the ground. There are two modes of hover flight - Hover in ground effect (HIGE) and hover out of ground effect (HOGE).
Hover In Ground Effect
Hover in ground effect is where you are close enough to the ground (around 4 feet) that you draw the air down through the rotor disk and it pushes in into the ground. In accordance with Newton's Third Law, as the downwash pushed into the ground, it also pushes back towards the rotor, creating a high pressure "bubble" beneath the helo, and reduces the recirculating air on the rotor tips.
Hover Out of Ground Effect
Hover out of ground effect is where we are roughly over a rotor span in the air - say around 50 feet. We have now lost that cushion of air beneath us because there is no air trapped between us and the ground. It simply recirculates around and around.
Sometimes a picture is worth a 1000 words:

As I hover in ground effect, I get a bit of help because I have that bubble helping keep me in the air, meaning my rotor has to do less work (really its less AOA or pitch pulled from my collective), whereas I don't have that help out of ground effect. The difference (at least for the bell 412) is around 11 percent torque. This means at a given take off weight, I will require 11 percent less torque in ground effect than out of ground effect to maintain hover flight.
Getting back to power required, this is the torque needed to lift the sum of all the weight on board my helo (and the helo itself). This means the more cargo + more fat bodies + more gas in the tanks = more power required. This is going to affect my HIGE and HOGE torques, along with my minimum safe airspeed (which i'll cover later).
Power Available
Power available is the second half - this is what the engines and rotor can produce in terms of power and thrust/lift. The 2 main factors are altitude and temperature. We measure these in terms of torque as well - mast torque for the rotor and engine torque for the motors - but for now we are only going to focus on mast torque. Essentially, this is the amount of final drive power we have in our pocket. Air is power - so the more air we have, the more torque we produce. Cold air at sea level in the Arctic will produce more power than hot air in the mountains of Afghanistan - see density altitude if you want to learn more. For a given altitude and temp, I will be able to produce a certain amount of power measured by rotor torque percent. A cold day I could get 100% mast torque. A hot day I might only get 85% mast torque because I've run out of engine turbine temp to produce power or I've run out of collective pitch because my rotor blades are at max pitch. Most helicopters are also multi-engine, so two engines running will get me 100% torque, where if I lose an engine I might only get 60% mast torque.
Finally  there is a safe minimum airspeed for single engine operations (there is the deadman curve for twin engine, but we are talking emergencies here). As a helo hovers, it draws air down from above to create the airflow over the blades - essentially the spinning of the rotor is solely responsible for creating relative wind. As we start to translate into forward flight, airflow is established over the blades by forward movement, creating a relative wind more in line with the blade chord. Our forward flight means the helicopter depends less on pure blade rotation to create air movement for lift because forward flight is providing the clean relative wind over the blades. This lets us stay in the air with less torque required.
If you made it this far, great!
Example
So how does this tie all together? Lets use an example:
All up weight: 11,500 lbs.
Fuel on board: 2000 lbs.
Mast torque available (twin engine): 100%.
Mast torque available (one engine inoperative - 2.5 minute power): 65%.
Mast torque available (one engine inoperative - continuous power): 54%.
Minimum airspeed (single engine): 25 kts.
Hover torque required (in ground effect) 75%.
Hover torque required (out of ground effect) 86%.
Helicopters landing at or near maximum all up weight is normally limited to a factor of power required/available rather than a factor of airframe loading limits/overstressing
Scenario #1
You are in ground effect hover and lose an engine. You pull the collective into your armpit and accept settling with power because your power available single engine (65%) cannot meet the power required (HIGE 75%). Fuel dumping is not an issue here.
Scenario #2
You are out of ground effect hover and lose an engine. Again, you pull the collective into your armpit and accept significant settling with power because - you guessed it - power available single engine (65%) cannot meet the power required (HOGE 86%) and you accelerate to the earth. The whole iteration takes about 5-7 seconds, and the result lands you as a flaming fireball on the 6 oclock news - however you don't get to see it. This is why you don't hover heavy OGE, and why you don't violate your deadmans curve! No time for fuel dumping, so not an issue here.
Scenario #3
You are in a high hover (HOGE) - say 150 feet. You lose an engine, but instead of pulling the collective into your armpit and dying, you pull some power and push the cyclic forward - accelerating above safe single engine speed (25 kts). You start to arrest your descent as you trade altitude for airspeed, bringing enough speed in to climb out safely. You can then either choose to stay in the air on single engine for the next hour to burn enough fuel to facilitate a HIGE landing, or you can carry out a run-on landing at or above 25 kts at your current weight. No fuel dumping required.
Scenario #4
You do a no-hover (or rolling) takeoff and climb out above minimum safe single engine speed (25 kts), and then lose an engine. You have two options - continue the climb out and burn some gas until you bring your all up weight down to a torque that will allow you to hover single engine 2.5 minutes (which will take at least an hour - thats why flight engineers bring ipads and magazines), or simply continue your circuit and carry out a run on landing (or rolling landing) at or above 25 kts. Nice, slow, not a lot of runway needed. Fuel dumping not an issue. Keep in mind that a nice slow decent with loaded blades is key - every approach is an approach to overshoot with the option to land.
Summary
Remember, the fuel loads in most helicopters - even the big ones - are measured in thousands of lbs and not tens of thousands of lbs. We don't carry a significant amount of gas to really alter our landing profile - all we need is enough to stay in the air for a couple of hours. After that, our backs, necks and egos are sore from terrain flight and vibrations.
As an extra note, trying to dump fuel in the hover would be a hazardous mistake. As you dump fuel mist, it would recirculate through the rotorwash back onto the helo - into the engines and all over the place in general. Then after it lights off, your land as soon as practical emergency just turned into a one bell alarm. IF it were to happen, you would need an established minimum airspeed to carry it out, but then you certainly wouldn't be transferring it to a ground storage tank.

Well, if in #3 and #4 it's hour to burn the fuel to facilitate  HIGE landing, fuel dumping would help. Especially for ship-based helicopters that can't do run-on landing because the landing pad is not large enough (note the accident RedGrittyBrick found was a ship-deployed helicopter).

For sure, absolutely a fuel dump (if able to be done safely) would help. I did not read the accident report, so I cannot comment specifically on what happened. I was not ship ops qualified, however if I were in a situation like that where a run on landing was required to a confined area, I would want that ship into wind and going as fast as it can.  The wind speed plus vessel speed should compensate for the 30 kts or less to maintain single engine safe speed - most of the time i'm in the 5-15 kts range. I'm not against fuel dumping, its just a compromise between design and requirements.

As one of the original commenters I suppose I'm as good as any to reply here!
I can't remember my wording off the top of my head, but essentially I pointed out that if a helicopter has the power to lift off at a certain weight, or hover/climb, it is by definition capable of slowly descending and therefore landing.
Unlike an airliner which has to essentially 'fly' onto the runway, straining the landing gear, a helicopter does not. In theory, one can land with virtually no extra force other than the weight of the heliopter. In practice this isn't quite true, but you get the idea.
Now in reality that doesn't mean a helicopter will never dump fuel (or want to) - in an emergency situation it makes sense to reduce weight to improve responsiveness, and if I'm about to crash I'd like as little highly flammable liquid around as possible!
There are also a small number of (mainly military) helicopters which do generate lift when moving, therefore can takeoff at a higher weight than they can hover... In these cases they are more akin to fixed wing aircraft in terms of having to 'fly' onto the ground rather than performing a vertical landing, and therefore may need to reduce weight to reduce stress,

Each and every helicopter can fly heavier than it can hover. It is a direct consequence of fundamental physical laws—like in fixed-wing aircraft, increasing speed reduces induced drag and therefore power required to maintain altitude.


text:
Why can't Norwegian's Boeing 787 operate from Vegas during hot weather?
In a German news article it was stated that the additional weight (Norwegian puts 291 passengers on their 787-8) leads to operations restrictions in Las Vegas when temperature conditions exceed 40°C.
Is the 787-8 really limited at 40°C? What would happen at airports that are not only hot but also high?
What are the temperature/altitude limitations for the 787-8 at MTOW in this specific case?

European low-cost carrier Norwegian Air will suspend its four Las Vegas routes in March, saying hot weather there has created performance issues for its heavily loaded Boeing 787 aircraft.
— usatoday.com
— aerotelegraph.com

NOT A DUPLICATE! This question asks abou the 787 & if the temperature limits are really as low as stated; the other question is a general discussion of how hot weather can be limiting, but it DOES NOT EVEN MENTION the 787.  Related, yes, but certainly not a duplicate.

I've attempted to reword the question to A) make it more specific based on the comments, and B) show that it's not a dupe of the suggested on. If I've taken it off course, please revert. Also, @OP, please quote the source of your German news article.

@FreeMan thx. I have added the german news article.


text:
Why is the empennage of some aircraft not placed on the centerline?
Check the MD80's empennage below.

Can easily tell it's not built on exactly centerline by naked eyes.
Is that because aluminum joint or consider aerodynamics?
I don't think it is manufacturing defects or imprecision.
Some of the small aircraft have the same construction like this.
Couldn't find the similar discussion online though.

Very nice website, thx :)

Looking at the picture, I see that the forward portion of the empennage fairing does not line up with what appears to be a joint in the skin structure. However, the joint may be off-center compared to the rivet placement. Looking at the empennage in person may reveal it to be off centerline, but this is not clear to me from the picture. Could you further explain or document how the empennage is not on centerline?

@Jonathan Walters Good point! I don't figure out that I should compare to the rivet placement. Joint line is not centerline clearly.

This is a good question, because some aircraft do have off-center (or divergent or angled from the longitudinal axis) empennage. The C172 is an example of such an aircraft; I'm not sure that the DC-9/MD-80 series is.

@Jonathan Walters Off-center empennage? For better aerodynamics or compensate the other problems? Could you give me more information about this?

I haven't had my hands on a C172 in a few months, but if I remember right, the built in angle is to counteract some of the left turning tendencies and provide built in trim. The C172 doesn't have cockpit-adjustable rudder trim.

@Jonathan Walters Oh, I thought manual control the rudder to counteract that turning tendencies on a C172 is the only way. Thx, that help a lot.

@ReinFore Rudder control is the primary way, but the built in angle and the ground adjustable trim tab also provide trim.

Another possible reason is the stability of yaw control. Some plane designs had the curious (and unintended) feature that perfectly symmetrical flight was unstable in yaw, but there were two stable attitudes yawing slightly (a fraction of a degree) left and right. Early autopilots sometimes "hunted" between those two stable states while trying to maintain the unstable symmetrical state, and the side-to-side oscillations could make passengers in the rear rows of seats feel a bit airsick. Intentionally breaking the "perfect" symmetry cured the problem.

You said "aircraft", not "planes", so you should look at helicopters because almost all of them are asymmetric by definition : )

Its my first time seeing this kind of defects. For sure its not a defect caused by manufactures. It could be that this aircraft had gone through a skin change procedures. Its the overlapping of two skins that appear to be not center or could be the dorsal fin's anchor nuts not rivets properly i guess so (refer to my experience working with b737-family).

this does not add anything to the already existing answer

Are you suggesting that this is a defect? Are you familiar with the construction of the MD-80 type? Is this, in fact, an example of defective manufacture? I have seen nothing else that suggests the photo in question depicts a defect; my understanding is that the photo shows proper construction that differs from the expectations of the OP.

I'm sure this section haven't been modified and other MD82 are the same. Have repairing experience on MD82/83.


text:
Why does this regional jet have its air brakes wide open before touchdown?
I was looking at a detailed picture of a plane while following its path on Flightradar24.com. Here is the Avro RJ45 I am talking about:

I wonder why its aerobrakes at the empennage seems wide opened before touchdown. What I remember from all the planes I took is flaps are at maximum before touchdown and aerobrakes (sometimes automatically) opens when landing gears touch the runway.
Is it due to special constraints for this particular aircraft?

I think you are confusing speed brakes with spoilers. Speed brakes are deployable in the air, usually up to Vne. Spoilers destroy lift and are usually interlocked with the squat switches so that they aren't inadvertently deployed in the air.

@RonBeyer, spoilers often are deployed in the air and act as air brakes, though usually they don't extend all the way then. They are not used at low speed though as they increase the stall speed, which speed brakes on other parts of the plane than wings don't.

Primary reason is speed control.  The thrust output of a jet engine does not have a linear relationship to the gas core or fan speed and the most effective thrust range is somewhere between 75-100% N1.  Commercial jets and military fighters tend to be quite 'slippery' and require drastic throttle reductions to slow down without the use of speedbrakes.  This puts the aircraft at considerable risk at lower altitudes or on short final where a large thrust increase may be needed to correct for a gust factor, wind shear or rapid descent.  Unfortunately it takes a period of time for the gas core and fan of a jet to 'spool up' and start providing the needed thrust.  This slow response time of early jet engines in critical power situations has produced horrifying results, as was the case here on July, 1955 with a fatal rampstrike of an F7U-3 Cutlass coming aboard USS Hancock (CV-17).  See at 5:43 into the video.

It can also happen even with the most modern engines as this F-18 rampstrike proves.

To compensate for the thrust response problems, pilots will fly the pattern and approach at a high thrust lever setting, then maintain speed and descent control using application of speed brakes.  This gives good energy management and control but allows the pilot to quickly get thrust if he needs it to return to glideslope, climb or go around altogether.

In the USS Hancock video, what's the reason for everyone landing with an open canopy? Visibility?

Dunno.  It's a good question.

Just posted it as a proper question :)

It used to be common for Navy pilots to fly propeller driven aircraft as well as early jets open cockpit during low speed flight.  The earlier canopy designs permitted this.  I don't think it was done for any specific reason save maybe they like the feel of flying that way.

The Fokker F-28/F70/F100 and the BAe 146/Avro RJ are the only two airliners with rear mounted air brakes because they were the only two airliners not designed with thrust reversers.
It is Standard Procedure to use the rear mounted air brakes on every landing because they add drag and reduce the landing distance.
Other airliners don't have this type of air brake simply because they don't need them.
These two types also employ wing spoilers for use on touchdown just like other large aircraft. Wing spoilers are only used after touchdown, or at higher altitudes, and are not used during the final approach and flare.

Yes but the Fokker 70/100 are developments of the F28 which had no thrust reversers. That is the only reason it retains the air brake.


text:
What is the reasoning behind using depleted uranium as counterweights in the 747?
I recently found out that some of the earlier models of 747 had between 600 and 1100lbs of depleted uranium as counterweights. These counterweights were found in 747s manufactured between 1968 and 1981.

(Source)
Given that the study linked above and the well-known radiation exposure for workers (and passengers) even prior to the 1968 introduction of these counterweights, why did Boeing (and McDonnell Douglas) decide to use such a hazardous (and potentially dangerous from a military perspective) material?
I understand that the weight/density allows for a smaller package, but it seems that the tungsten weights were integrated after 1981 without any space issues in existing aircraft.  Was depleted uranium in such high supply that it was also more economical?
Eventually, the counterweights were replaced with these tungsten replacements:

(Source)

DU is only hazardous when you come into contact with  particles; especially when inhaled.  In a crash severe enough to compromise the integrity of the weights; this would be your last concern. My guess is that a more safety conscious (litigious) industry and availability of alternatives led to other materials being used.

This was also found on the L-1011.

@Simon : and in this regard DU is not in any way more hazardous than any other heavy metal, like lead or tungsten. Why it is falling out of fashion today is not because of safety concerns, but because of the hysterical public thinks "uranium = dangerous radiation or nuclear explosion hazard". However, depleted uranium, and even regular (non-enriched) uranium emits so little radiation that it's completely negligible. In fact, uranium can be used to shield against radiation (it's much denser than lead). You'll probably get more radiation from eating a banana than holding a piece of uranium.

@vsz: The (much) greater safety concern is that uranium has very potent chemical toxicity, much greater than that of, say, tungsten.  Also, while DU is quite good at shielding against most ionising radiation, it isn't an option for anything producing significant quantities of fast neutrons, as these fission the uranium atoms and produce lots of secondary radiation.

People who are dealing with the aftermath of a crash and exposed to the clean-up were not 'in' the crash, so yes, following a crash severe enough to compromise the integrity of the weights poisons released by the crash is very much a concern. It's one of the reasons if you see pictures of people investigating the wreckage they will usually be wearing PPE, at least masks and gloves, sometimes bunny suits.

Boeing
Boeing would have used DU because it had the right combination of physical characteristics and cost. Their tests showed that the radiation exposure for workers was low (2.6% of the statutory "safe" level). In most cases the exposure was so low as to be not detectable.

Radiation
Passengers on aircraft are exposed to cosmic radiation at much higher levels than people on the ground.
On the ground the average American is exposed to 620 mrem/year from all sources.
Those flying from Washington DC to Los Angeles would be exposed to close to 2 mrem from cosmic radiation. This is not an issue for passengers but is something that airline crew and other very-frequent flyers are aware of.
Exposure to aircraft occupants from cosmic radiation is 600 times higher than from DU counterweights in the early 747 aircraft.

Depleted Uranium
DU is Uranium from which the most radioactive parts have been removed. It is less radioactive than naturally occurring Uranium.
Heavy metals like Lead, Tungsten and Uranium are toxic if ingested. This toxicity is not due to radioactivity. Aircraft crew and passengers are not exposed to these metals in any way that could cause them to be ingested.

Boeing Tests
Boeing carried out safety tests on the material.
Workers

Boeing
  has
  conducted
  two
  dosimetric
  studies
  of
  exposures
  to
  workers.
  For
  both
  studies,
  whole
  body
  exposures
  were
  measured
  with
  film
  badges
  which
  were
  provided
  and
  processed
  by
  Landauer,
  Inc.
  These
  badges
  have
  a minimum
  detectable
  exposure
  of
  10
  millirem
  per
  issue
  period
  (monthly).
  In
  the
  second
  study,
  extremity
  exposures
  were
  measured
  with
  finger
  rings,
  also
  provided
  by
  Landauer,
  having
  a
  minimum
  detectable
  exposure
  of
  30
  millirem
  per
  issue
  period
  (also
  monthly).
  The
  periods
  of
  the
  study
  were
  December
  1968
  to
  February
  1970
  for
  the
  first
  study
  and
  September
  1977
  to
  April
  1978
  for
  the
  second.
Both
  of
  these
  studies
  showed
  all
  worker
  whole
  body
  exposures
  were
  less
  than
  2.6%
  of
  the
  exposure
  limits
  for
  occupationally
  exposed
  employees
  (5000
  millirem
  per
  year)
  and
  less
  than
  26%
  of
  the
  limits
  for
  members
  of
  the
  general
  public
  in
  effect
  at
  that
  time
  (500
  millirem
  per
  year).

Crew and Passengers

Based
  on
  the
  data
  from
  National
  Lead,
  reported
  in
  the
  section
  on
  exposures
  to
  the
  general
  public,
  dose
  rates
  to
  flight
  crew
  will
  be
  less
  than
  0.8
  microrem
  per
  hour.
  During
  a
  2000
  hour
  working
  year,
  this
  results
  in
  a
  maximum
  potential
  exposure
  of
  1.6
  millirem,
  less
  than
  2%
  of
  the
  100
  millirem
  per
  year
  limit
  for
  members
  of
  the
  general
  public.
  This
  is
  only
  1/600th
  of
  the
  500
  microrem
  per
  hour
  increase
  in
  dose
  rate
  from
  cosmic
  radiation
  flight
  crew
  experience
  at
  39,000
  feet.

Boeing Use of Depleted Uranium Counterweights in Aircraft. 1984. Retrieved from NRC

References

Space Radiation, Should Frequent Flyers Worry? 2013. BBC
Tungsten Poisoning. 2014. Retrieved from Patient.info

It still seems like a lot of effort to go through, studies, exposure tests, etc to save a few dimensions or dollars over tungsten. Eventually Boeing and MD decided that the risk was worth removing the counter weights so I can't agree that it was found entirely safe. In the ELAL 1862 crash something like 150kg of DU was never recovered.

@RonBeyer Perception of safety matters almost as much as actual safety to an airline. They may have removed it because the public perceived it to be unsafe, rather than because it was actually unsafe.

The 1992 El Al incident highlighted the risk to rescuers. There is no significant risk to workers or passengers, which is what Boeing had checked, but while correct that still is an incomplete conclusion.

TL;DR just because it's called uranium doesn't mean it's dangerous. Soldiers sit in vehicles "made of" (ish, a bit) depleted uranium, next to stacks of ammunition made of depleted uranium. And it turns out it's more radiologically perilous to just fly in a plane at all. (Or, possibly, lay on a beach without sunscreen.)

@GrimmTheOpiner: Ironically, tanks and APCs are about the only places where you would have reason to shy away from using DU; in a nuclear attack, DU-armoured vehicles would become deathtraps for their occupants, due to the high fast-neutron flux inducing fission reactions in the uranium, releasing lots and lots of secondary radiation in the process.

[W]hy did Boeing (and McDonnell Douglas) decide to use such a hazardous (and potentially dangerous from a military perspective) material?

Because it's not particularly hazardous and it's not very dangerous from a military perspective.
Uranium comes as a mixture of two major forms (called isotopes). Uranium-235 accounts for about 0.7% of the mix and is the isotope that supports nuclear chain reactions so is used in nuclear fuel and atomic bombs. (Actually, almost all nuclear weapons since the bomb dropped on Hiroshima have used plutonium but that's a different story.) Almost all of the rest is Uranium-238, which is only mildly radioactive. It doesn't support nuclear chain reactions, so it can't be used in a bomb. Depleted uranium (DU) is uranium that has been processed to remove almost all of the Uranium-235. In other words, DU is almost entirely U-238.
U-238's radioactivity is mild in two senses. First, it has an extremely long half-life of about 4.5 billion years, which is about the age of the earth. Second, most of the radiation it emits is alpha particles. Alpha particles are trivial to stop with even the most negligible "shielding": a piece of paper or even a few centimetres of air is enough. Human skin is good shielding against alpha radiation: the particles are stopped by the outer layers of the skin, which are already dead, and which are shed in a few days anyway. You're not going to get skin cancer from low-intensity alpha-sources.
U-238 is only really hazardous to health if it gets inside your body. When that happens, it's bad for you in two ways. First, it's toxic in much the same way as any other heavy metal. Second, if it's inside your body, then the alpha particles it emits are hitting living cells of your body, rather than your dead skin, so now it can cause cancers. Inhaling dust is a big risk, here, and that's why soldiers who've worked with DU weaponry can have problems. In the case of the counterweights, this risk is mitigated by coating the DU so people aren't coming into contact with it.
The military implications are small.  DU has two main military uses: nuclear weapons ("Waaaait, I thought you said it couldn't be used for that!") and non-nuclear weapons.
Because of its density and the fact that it burns vigorously when finely powdered and exposed to the air, DU is used in some anti-tank weapons. According to Wikipedia, the coalition used more than 1000 tons of DU-based non-nuclear weapons in Iraq in a three-week period in 2003. Each 747 contained between a third of a ton and half a ton of DU, which corresponds to about ten to fifteen minutes' worth of ammunition. That doesn't seem to be a big concern.
Application of DU to nuclear weapons is, like everything else in this answer, in two ways. First, it's used in the "tamper" of a nuclear weapon, which reflects neutrons back into the weapon's core and allows smaller weapons to be made. Second, you can put U-238 in the right kind of nuclear reactor and make the exact kind of plutonium that's used for bombs. However, neither of these is a big concern because any organization that's capable of turning DU into nuclear weapons must already have enough infrastructure that they will already have their own large supplies of DU and not need to pillage it from 747s. In particular, they must already have nuclear reactors to convert DU to plutonium and isotope separators to extract the plutonium.

DU is also used in ammunition without refinement. Either way at some point (as in my comment on the other answer), Boeing and MD decided to not only stop using the DU counter weights, but remove them from older aircraft, so somebody decided that there was enough of a concern to go through that process.

@RonBeyer: "DU without refinement" doesn't make sense; refinement or enrichment is what splits natural Uranium into Enriched Uranium and Depleted Uranium.

@MSalters I meant without further refinement. I understand there is a process to create DU, however it doesn't need a lot of further processing to be used in that regard.

@RonBeyer I don't really understand. Once you've made DU, you have DU. You don't need to refine it any more because it already is what it is: depleted uranium.

The DU tamper of a nuclear weapon doesn't just serve as a neutron reflector; if it's a thermonuclear weapon (as essentially all nukes are nowadays), a large portion of the total explosive yield comes from the fission of the DU tamper by the fast neutrons generated by fusion.

Depleted Uranium is 68% denser than lead and costs much much less as DU is a discarded material.  Therefore, there were both space requirement considerations as well as cost considerations when using DU as a ballast or counterweight in aircraft.
Although precautions are taken, there are no substantial health concerns when using DU in this way.  The radioactivity of the DU is no longer a factor as it has been depleted.
Former Fighter Pilot / Private Jet / Corporate Jet Pilot

Googling for "cost of lead" and "cost of depleted uranium" suggests that lead costs about costs about $1/lb, compared to $5/lb for DU. So, although bigger, lead counterweights would be significantly cheaper. DU's much cheaper than tungsten ($25-45/lb) and easier to machine, but it's plenty more expensive than lead.

+1 for Density, which is the key design factor. It is not just flutter mitigation, but harmonic damping (or snubbing) that has to be achieved by internal placement of the DU, sometimes at extremities where space is at a premium. DU makes a far more compact flywheel than lead, for the same reason. I agree the ionising radiation risk is negligible considering the background counts at altitude.

So you're not going to edit your claim that depleted uranium is "costs much much less" than lead, even though it's actually five times more expensive?

TBH you'd have to check the historical prices.

@DavidRicherby: Some more googling on my part suggests that, in 1968 (when the DU counterweights were first installed), depleted uranium had a lower price per unit mass than lead.

I'd say reason for using non-radioactive Uranium in counterweights is same as in perforating bullets: it has one of the highest specific gravity available, and this means a high weight, a higher inertia, momentum, within same volume, compared to other materials, including Lead.

The question already states that DU has a very high density. What does your answer add to that? (And, by the way, DU is still slightly radioactive.)

I wanted adding examples of Depleted Uranium use with the same rationale, but please feel free to delete my entry. Depleted Uranium is not radioactive, at least no more than an ordinary granite soil, your observation may come from activists who overacted regarding dangers of Uranium in perforating bombs and other warfare, but we all know that, as G Bush said: 'War is a dangerous place', no need to remark remote and long acting combat risks for those fighting. Do you have a control-review command from the site?

@Urquiola: DU from munitions is dangerous to those exposed to it, but this is almost entirely due to its considerable chemical toxicity, rather than its (extremely mild) radioactivity.

I'm not that sure. The 1972 edition of OIT: 'Encyclopedia of Medicine, Hygiene, and Safety at Work', tells, about use of Uranium in catalysts: 'Radiation risks from catalyzers are low. The Uranium radiation is of low penetration, no need exists to provide protective screens. The Uranium used is not soluble, and the risk of chemical intoxication is small. Cautions must be addressed against retention of  non-soluble Uranium inside lungs and protection of skin against the surface radiation'


text:
Do ETOPS requirements vary between passenger, cargo, and non-revenue flights?
Are there different ETOPS requirements for aircraft depending on usage?  For example, does a FedEx cargo flight need the same ETOPS ratings for over ocean flights as a passenger flight?  Also, how about ferry or repositioning flights which are not revenue flights?

Related: What is ETOPS and how does it work?

The ETOPS requirement is pretty much the same for all aircraft irrespective of usage as long as its under parts 121 or 135, as noted in the FAA InFO 07004, New ETOPS Regulations:

The new ETOPS rules are now applicable to:
Part 121
• All two-engine airplanes more than 60 minutes flying time from an adequate airport (at a one-engine-inoperative cruise speed under standard conditions in still air).
• All passenger-carrying airplanes with more than two engines and more than
  180 minutes flying time from an adequate airport (at a one-engine-inoperative
  cruise speed under standard conditions in still air).
Part 135
• All multi-engine turbine-engine powered airplanes (other than all-cargo
  operations with airplanes having more than two engines) more than 180
  minutes flying time (limited to 240 minutes flying time) from an airport
  meeting the requirements of part 135, §§ 135.385 or 135.393 and 135.219 at
  an approved one-engine-inoperative cruise speed under standard conditions in
  still air.

As far as differences are concerned, ETOPS is waived only in one case:

The FAA ... however agree that for airplanes with more than two engines, passenger carrying operations may be excluded from the ETOPS maintenance program requirements and that all-cargo operations may be excluded from all ETOPS requirements.

The definition of ETOPS in the same document covers all aircraft except four engined all-cargo aircraft:

Extended Operations (ETOPS) means an airplane flight operation other than an all-cargo operation in an airplane with more than two engines during which a portion of the flight is conducted beyond a time threshold identified in part 121 or part 135 of this chapter that is determined using an approved one-engine-inoperative cruise speed under standard atmospheric conditions in still air.

As far as non-revenue flights are considered, they are usually operated under Part 91, so the ETOPS rules don't apply. However, it is better to plan an ETOPS route for safety considerations (and to keep insurers happy).

The wording makes it look like cargo flights with three, or five or more, engines would also be exempt...


text:
What is required by and for flights that cross borders in terms of border control?
This can apply to countries that have water in between, like the UK-France or US-Canada.

If an aircraft crosses into neighbouring country, does it need to go to a border-controlled airport (with flight plan and access granted)?

An IFR-filed flight with permission from that country's civil authorities, but to a non-international airport.

Are international airports the only border-controlled airports or do some countries have predetermined airports that pilots can fly to, so that border control authorities can process them at said airport?

A famous US/Canada example is Niagara Falls.
As as student, part of my cross-country test involved flying a circuit around the Falls, which meant passing the international border.  No special permission was required.
If I'd gone anywhere near the American airport (top right of map), the situation would have been completely different though.
Note that even though there are no specific border control related rules, there are many safety regulations controlling this circuit.  (It's a lot more crowded now than when I flew it.)

GA Flying over Niagara Falls – Fly 'n Things

@ymb1, The special rules are related to navigation for purposes of safety, and worth mentioning. but as far as I know there are no rules specifically related to "border control".

The details will vary by country, but in general, if you cross a border, you will need to get permission from one or both countries, file a flight plan and be talking to ATC/FIS as you cross. If you want to land in another country, you will need to do so at a designated port of entry. In some countries, you must also leave from a port of entry.
The term "international airport" doesn't have any specific legal meaning but is generally used by airports that have (or had, or hope to have) scheduled commercial flights to/from other countries. Many smaller airports are ports of entry too, either full time, part time or by appointment.


text:
How realistic is the plane scene in Captain America: The First Avenger?
In Captain America: The First Avenger, there's a scene in which Captain America gets into the villain's plane by holding onto the landing gear as it's taking off and waiting for it to be retracted. How realistic is this scene, when applied to modern airplanes?
Could you theoretically gain access to a large, modern airplane by just holding onto the landing gear - assuming, of course, you managed to hold on? Do the landing gears of large modern planes retract into interiors that have access to the rest of the plane, or do those interiors tend to be sealed off? And for that matter, how hard would it be to hold on to a plane's landing gear as it's taking off?

And on our site we usually prefer that you only ask one questions at a time...  But in summary: People have snuck into aircraft landing gear, some have survived, some have not.  But, to my knowledge, no modern airliner has access from the landing gear to the cockpit.

See also: Commando (1985), where Arnie gets out of the plane via the wheel well.

Nothing is realistic about airplanes in Hollywood.

Regarding holding onto the landing gear as the aircraft takes off, or otherwise getting into the wheel well, stowaways do try this on a regular basis.
As other answers have said though, they then find that there's no access through to the rest of the plane, so they're stuck in the wheel well.  At cruising altitude (above 30,000ft) the air temperature will typically be below -50degC.  Surviving at this temperature needs specialist clothing and breathing apparatus, which stowaways usually don't have.  The result is that they usually freeze to death.  Perhaps mercifully, they'll lose consciousness above about 10-15,000ft due to lack of oxygen, so they won't experience this happening to them.
At major international airports such as Heathrow, there are regular cases of bodies found in fields (or even in gardens) along the flightpath, as aircraft drop their landing gear and the frozen stowaway falls out.
Occasionally they do survive.  Generally this will be because they've wrapped up warm before they leave.  The aircraft tyres can also be warm, which helps too.  And it's more likely on a shorter flight, of course.  It's pretty rare though.

This doesn't seem to address the issue of access between the wheel well and aircraft interior at all.

@MichaelKjörling it does, but only in an extremely marginal way, while it should be the focus of the answer.

@MichaelKjörling The OP asked "And for that matter, how hard would it be to hold on to landing gear as a plane is taking off?"  Evidence shows that people do manage it.  Evidence also shows what happens to them.

Do these stowaways really hold on to the landing gear during take-off, or do they hide in the wheel well much earlier?

Though 'getting on/off aircraft through landing gear' is a favorite trope in movies, it is practically impossible in most of the modern airliners. Airliners are pressurized, and as a result, the cabin (and cockpit) area is in reality one big cylinder, cut off from everything else. So, no accessibility from the landing gear area.
For example, the following image shows the landing gear bay (the area into which the landing gear retracts into) of a Boeing 787.

Image from dailymail.co.uk
As a result, though the list of stowaways in landing gear area is long, to the best of my knowledge, no one has entered the aircraft through it.

While this is a good answer, I feel like referring to an airliner as a sealed cylinder or a "pressure vessel" (Carlo's answer) is misleading.

@Coxy It is entirely accurate: The main body of an airliner is a pressure vessel - otherwise it would not be possible to pressurize the cabin for high altitude flight. It is not perfectly cylindrical - there are indents in the shape that allow the nose gear (among other things to stow) within the outer fuselage shape - but there is generally not a penetration at these points, and the wall in these areas is shaped specifically to withstand pressurization at altitude.

But "sealed" would suggest that no air gets in or out, and this is not the case, thought it's a common misconception.  Pressurization doesn't require a perfectly sealed vessel - just the ability to pump air in at least as fast as it leaks out.  There are penetrations in this cylinder - notably the outflow valves - though admittedly they are much too small for a person.  Obviously you know this, but it seems like the answer as it stands tends to perpetuate the misconception.

Another view of a B787-8 bay from Rudi Boigelot and details of a A340-600 gear.

I think Aeroalias and @voretaq7 are quite right in the principle, but Coxy and Nate comments are very valid and require more details to be provided related to existing openings in this pressure vessel, their location, their size and how they could be used (opens with a handle, a screwdriver, a wrench...). A missing window could be handled by the packs, so the scenario is not impossible. This may be difficult to answer without working in the maintenance department of an airline. The question is a good one!

@NateEldredge For the purposes of this discussion. Any hole big enough for the Capt, would require an unrealistically powerful pump to maintain the dynamic pressure. In fact, it would almost be making the entire cabin into a jet...

However, some aircraft do have an access hatch through the nose wheel well (747 IIRC). However once the plane pressurises, it would not be possible to open it over the pressure differential. And the pressurisation starts sometime during the take-off roll. It might be possible in such a plane during taxi though.


text:
Why do cockpits with ACARS have printers?
Most cockpits with ACARS have a thermal printer that is used for printing out information received.  Why is this information printed out rather than displayed on a scrolling computer screen?  Maybe I'm not used to ACARS or too used to 21st century technology where all messages and documents are on a screen, but it seems like the flight plan, maintenance, and weather information transmitted over ACARS could be saved and read off a screen as easily as a piece of paper.
What are the advantages of having a printer in the cockpit for ACARS and other systems, and is this technology being replaced at all in modern glass cockpits?

Edit:  As originally written this question didn't address other information that could be printed by advanced printers like charts and weather maps, but if you feel this is a relevant detail, you can include other systems other than ACARS in your answer.

Not all cockpits with ACARS have a printer for it -- it's an entirely optional part of the system.
The printer is there so that you don't have to copy by hand information that you want or need to have available.  For example, ATIS is desirable to have in hard-copy, so that when you need some piece of it immediately (say, the altimeter as you're descending through the transition level, or the identifier as you go to check in with Approach), it's there without going through the levels of menus to retrieve it from the ACARS system.
Also, on a Class II navigation route, if you get a significant reroute, your procedures may dictate that you have a hard-copy flight plan that shows the new route, and which you'll annotate with things like actual times & fuel onboard overhead various waypoints, actual winds, etc.  That may be required by company procedures or regulation, and the hard copy is there for those times when the computer decides to misbehave.  And all of THAT info is most certainly something you don't want to have to copy by hand -- it's pretty lengthy, and accuracy is really, really important.
Beyond that, it is "nice to have" a hard copy of things like a PDC clearance, and it is quicker to tap "PRINT" rather than copy it all down by hand.


text:
What is the relationship between the UH60 Blackhawk stabilator function lateral sideslip and pitch coupling?
I'm having a hard time understanding the correlation between the wind gusts (where are they coming from, what are they affecting) and why, in a nose left situation,  the stabilator is driven down.
I keep thinking it would make sense for it to go up (e.g. left pedal, more tail rotor torque, tail up, nose down.

sideslip to pitch coupling to reduce susceptibility to gusts.    When the helicopter is out of trim in a slip or skid, pitch excursions > are also induced as a result of the canted tail rotor and downwash on the stabilator.
Lateral accelerometers sense this out of trim condition and signal the stabilator amplifiers to compensate for the pitch attitude change (called lateral to sideslip to pitch coupling).
Nose left (right slip) results in the trailing edge programming down.
  Nose right produces the opposite stabilator reaction.

If the stabilator drives down, this will make the nose go down further.
I know I'm missing something big here but can't put it together.

von der tann. what is not useful or unclear about the question... also I did not come here for a grammar lesson.

Nothing is clear about this question. I have absolutely no idea what you're asking!

This feels like it could be a good question if it is properly edited.  Do you have any time to stop by our chat room and maybe explain the question in detail?  We may be able to help you rephrase it so you can get a better answer. http://chat.stackexchange.com/rooms/12036/the-hangar

@Jamiec Is it then a requirement that everyone should understand all questions or they should be closed?  I understand it, and have edited in an attempt to make it clearer for more people but in my view, this is a good question.

Hi.  Welcome to Aviation.SE. I've had a go at an edit, let me know if it works.  One thing I don't get.  What is your reference for the stabilator programming down with the quartering wind on the left of the nose (which I assume is what you are asking).  Do you have a link? It would also help to explain why the nose falls when the tr tq is reduced.  I don't follow that but, I'm not qualified on anything meatier than a B206.  Is it due to some aerodynamic interaction between the tr and the stab?  I also apologise for the rude welcome.  Some people forget what it's like to post a first question.

BTW, I am assuming, perhaps wrongly, that the FADEC will increase power in response to the tr tq going up and that the nose down effect is not caused by a reduction in mr torque.

From this answer, it appears that Mike is puzzling out this feature of the stabilator on the UH-60 Provide sideslip to pitch coupling to reduce susceptibility to gusts.

Mike, I have edited in a fuller description of how the system works.  What your questions seems to me is "why does it do that?" since you feel that it is counterintuitive? Right?

unfortunately, @JayCarr, with only 11 points, OP can't join us in chat... :(

@KorvinStarmast Ah, that makes sense now.  I've approved the edit but I don't quite have enough rep for it to stick without another vote.  This question also needs to be re-opened, it's ridiculous that it was closed in the first place.

@FreeMan Yeah, I realized it a little while after posting the link...

@Simon I think we can safely say that a reasonable person, who is reasonably knowledgable with a certain amount of Aviation knowledge should understand all questions, yes. I pretty much fall into that category. It seems 4 other people agreed with me - I dont have unilateral ability to close questions you know! This question, at the time I VtC was both badly written, and was entirely unclear. Now it is much better, and as such I have VtRO

@Jamiec Perhaps one for meta? "I think we can safely say that a reasonable person, who is reasonably knowledgable with a certain amount of Aviation knowledge should understand all questions, yes"  -  I meet the criteria, but I do not understand all fixed wing questions, nor do I vote to close them because of this.

The sideslip to pitch coupling is a result of the canted tail rotor and the asymmetry of the main rotor downwash. The tail rotor thrust pushes the tail to the right and upward (pitch down). The main rotor downwash across the tail boom also generates a pitch up motion.  The 'static' position of the stabilator is set (stab range is 10 deg up to 45 deg down based on airspeed) to counter this pitch tendency.
A right sideslip condition will result in a reduction of tail rotor vertical thrust resulting in a pitch up motion. The stabilator is programmed downward to counter the pitch up. Conversely a left sideslip will result in increased tail rotor vertical thrust and a pitch down motion.  In this case the stabilator will be programmed up to counter the downward pitch.


text:
Is the Boeing 737 unusually susceptible to stress fatigue?
According to this Newsweek article - which seems like a reputable source - the FAA has issued a number of Airworthiness Directives to address stress fatigue concerns with the 737NG series:

Between 2000 and 2011, as the number of NG series in the air grew to
  more than 2,000, I identified 13 directives that specifically
  concerned cracking and fatigue issues in the model’s fuselage.

The article focuses on potential issues with the aft pressure bulkhead:

But the most alarming alert concerned involved a repeatedly troubled
  area called the aft pressure bulkhead (the rear portion of the cabin).
  In an Airworthiness Directive dated Nov. 5, 2001, the agency called
  for “immediate corrective action” to deal with a flaw in the aft
  pressure bulkhead [...] Four airlines had discovered, during
  checks, that if a 737 made a hard landing, damaging the main landing
  gear, or simply a certain kind of hard landing that led to shimmying,
  involving violent swerves on the runway (not uncommon in turbulent
  weather), the forces transmitted from that impact would end up
  damaging the aft pressure bulkhead, fatally jeopardizing the tailfin.
There were five alerts about problems with this bulkhead. [Prof. Tony]
  Ingraffea, after looking at the [details], told me in an
  email: “Skin cracking and aft bulkhead problems are clearly chronic.”
  [...]
  In a written memo to Newsweek,
  Boeing claimed that to say that there were “persistent problems” with
  the bulkhead is “not correct” but did not elaborate.

Based on the number of ADs (and any other relevant information), is Newsweek accurate in claiming that the 737 is more susceptible to stress fatigue than other, similar aircraft? Is the number of ADs for this issue unusual? Is there any measurably higher risk associated with flying in a 737 because of this issue?

A comment discussion regarding this question has been moved to chat.

737s tend to be used for short, frequent flights, so they accumulate fatigue cycles faster than longer-haul airplanes.  Any answer will need to take this into account.

I suppose a simple answer is "is 13 AD/2000 aircraft flying" high, low, or average. Of course, someone would have to do the research...

One should not compare AD/number of planes, the relevant ratio is AD/aircraft type. Number of aircraft tells you how many people have been put at risk by underperforming designer/manufacturer, AD/type tells how competent the d/m organization is. Number of cycles and/or operating conditions are also not a very good excuse for AD's, as those factors should be taken into account during design phase. Of course the operator is responsible for careful handling of the planes...